Species in the order Mytiloida are found world-wide, but they are more abundant in colder seas, where they often form uninterrupted beds on rocky shores in the intertidal zone and the shallow subtidal. The subfamily Bathymodiolinae is found in deep-sea habitats.

Mytilids include the well-known edible sea mussels.

A common feature of the shells of mussels is an asymmetrical shell which has a thick, adherent periostracum. The animals attach themselves to a solid substrate using a byssus.